March 26, 2004: On other weather sites I have seen “streamline” depictions where lines are drawn parallel to the wind. When the lines come together, is that the same as “convergence”?
DrJack sez: What you are looking at is termed confluence and is not the same as convergence. Convergence does tend to occur where there is confluence, but they are really different animals. In fact, it is possible to have a confluent flow which has no convergence at all and thus no vertical motion! When one sees streamlines coming together one imagines the air will be “squeezed” together and forced upward due to the need for mass conservation – and that will indeed tend to occur. But the wind also tends to speed up in such a region and that by itself tends to satisfy mass continuity and thus reduce the amount of upward motion. It is possible for the speed up to exactly compensate for the confluence and so there need be no vertical motion at all (and no convergence). And in certain cases there can be downward motion in a confluent region – for example, when air must flow through a narrow gap in a ridgeline. Still, it is more common to have some upward motion (and thus convergence) in a
